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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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MILES AND GBJNNELL GLACIERS                           421
the mountain valley; but the northern edge merges into the moraine-covered piedmont bulb. The ice cliff is irregular, with great capes and coves, but it is not as high as the cliff of Childs Glacier. The form of the ice cliff has varied in detail throughout the period of observations and there have been several changes of moderate amount.
The Bulb and the Marginal Channel. The second or bulb portion of Miles Glacier is represented by the stagnant northern part, and by the lake basin from which the bulb has melted away. The stagnant northern part of the bulb still extends clear across the valley, as it did at the time of the last great advance when Copper River was forced over into the marginal channel (PI. CLVIII, A) which it now occupies. This northern portion, which rises to a height of 430 feet above the lake, or 500 to 555 feet above sea level, is all covered with ablation moraine, on which is a continuous alder growth, except where, as the ice melts, slumping has undermined the vegetation and let the moraine slide off the ice. Many small patches of this nature opposite Abercrombie Rapids were seen by us in 1909-11 and some also show in the woodcut from Aber-crombie's 1884 photograph. The river has been held in the marginal channel through lack of retreat of this portion of the glacier, which is of course much retarded by the mantle of ablation moraine and vegetation. Many enormous glacial bowlders skirt the river bank next the glacier, evidently helping to keep the river in its channel. At one point a cove, known locally as "The Bearhole," was formed in the glacier by slumping between 1884 and 1898.
When the glacier melts completely out of Copper River valley the site of the rapids will be marked by an abandoned marginal channel, perhaps cut somewhat into the rock, perhaps with one side missing, where the ice-wall now rises. It is, however, possible that in the meantime the river may so entrench itself in a rock gorge at Abercrombie Rapids that it will be unable to regain the middle of the valley after the stagnant northern part of the glacier melts away. It is not known how rapidly the river is cutting, but it flows with great velocity (PI. CLX), is heavily charged with sediment, and receives additional load from the rock and ice walls. At the foot of the rapids it has carried away all of the moraine left by the melting ice except a few of the largest bowlders.
The stagnant northern portion of the bulb has remained in about the same condition from some time before 1884 to the present time.
There are three quite different zones in the moraine-covered bulb which were studied in detail in 1910, and their relative areas represented on the map. The characteristics of these three parts are stated in the following paragraphs:
Zone of Thickest Ablation Moraine. The western or outermost part of the bulb (Thickest Ablation Moraine, Fig. 68) is deeply covered with ablation moraine, well seen in the portions facing Abercrombie Rapids (PI. CLX). This moraine supports a dense growth of vegetation (PL CLXI) with trees estimated to be about 50 years old, and with only scattered small areas where the underlying ice is revealed by slumping; but ice was still present in 1910, close to the bank of the Copper River at Abercrombie Rapids (PL CLVII), and along the northern border of the bulb opposite Allen Glacier. Abercrombie's photograph, taken 26 years before our visit, shows as dense a growth of vegetation on this portion of the bulb as at present.
Zone of Thick Ablation Moraine. The second zone (PL CLXV) is also deeply buried by morainic debris underlain by ice. Here the ice is inferred to be less thickly covered